Active noise-reducing headsets are used in many environments to reduce the level of background noise to which a user is subjected. For example, active noise-reducing headsets are commonly used in airplanes and other high ambient noise environments. Noise-reducing headsets are also used in home and office environments where a user desires more quiet. For example, a user may wish to listen to music or perform work without being bothered by background noise.
Active noise-reducing headphones take advantage of the ability of sound waves to interfere destructively with one another. Destructive interference occurs whenever a part of a sound wave that is in compression meets a part of another sound wave that is in rarefaction. The two sound waves tend to cancel each other out at that point, reducing the total amount of audio power at the point of cancellation. The maximum reduction in sound occurs when the peak of one sound wave (i.e. the point of greatest compression) meets the trough of another wave (i.e. the point of greatest rarefaction).
Active noise-reducing headsets use one or more microphones to detect ambient noise. The detected noise signal is phase shifted by 180° and played into the headphones as a noise-cancellation signal. The user hears both the ambient noise and the noise-cancellation signal. The ambient noise and the noise cancellation signal interfere destructively at the user's ear, which reduces the level of noise actually heard by the user of the headphones. Active noise-reducing headsets can include headphones that fit over and surround the ear, headphones that sit on the ear, and headphones that fit within the ear.
Some noise-reducing headsets can reduce background noise by as much as 50 dB. While more noise reduction is generally considered preferable, too much reduction in ambient noise can cause a user not to hear important audible information clearly, such as announcements, alarms, ring tones, and the like.